Solid-state power amplifiers have received considerable attention since the invention of the solid-state transistor and the solid-state integrated circuit. Heat is generated in the active regions of field effect transistors (FETs) and bipolar junction transistors (BJTs) as a result of I.sup.2 R losses. This heat affects the reliability and thus the mean time before failure (MTBF) of these transistors. Fourier's law of heat conduction describes the spatial flow of heat through a substance by the following one-dimensional expression: ##EQU1## where q, the heat flux, is a heat rate per unit area; k is the thermal conductivity of the conducting medium; T is the temperature; and d/dx is the differential with respect to distance. The more general form is the vector expression: EQU q=-k t (2)
where overstruck quantities represent vector quantities and the symbol represents divergence.
The semiconductor chip in which a solid-state power amplifier is built is typically mounted on a metal or composite material having a thermal conductivity that is at least two times greater than the thermal conductivity of the semiconductor, k.sub.semi. As a result, k.sub.semi is generally the limiting component in heat dissipation from the power amplifier.
The thermal conductivity of the semiconductor, k.sub.semi, is a function of temperature. For temperatures above approximately 20K, k.sub.semi can be expressed by the following relation: ##EQU2## where k.sub.0 is the thermal conductivity of the conducting medium at the reference temperature, T.sub.0 is the reference temperature, and T is the temperature of the conducting medium. The reference temperature, T.sub.o, is generally room temperature or, in degrees Kelvin, approximately 300K. For pure silicon (Si) and gallium arsenide (GaAs), the values for k.sub.0 are 1.45.+-.0.05 and 0.44.+-.0.04 W/cm-K, respectively.
When semiconductor impurity (i.e. dopant) concentrations exceed 10.sup.15 atoms/cm.sup.3, the thermal conductivity of the conducting medium is reduced. This results from photon-electron scattering. In typical semiconductor devices, k.sub.semi will not degrade more than approximately 20% due to semiconductor impurities.
Reliability studies indicate that the active temperature for semiconductor regions, T.sub.active, should not exceed about 150.degree. C. Ambient operating temperatures of any semiconductors are approximately 85.degree. C. Because of these limitations, the density of rf power generated by a power amplifier transistor must be maintained below a predetermined threshold, P.sub.Dmax, to maintain an acceptable operating temperatures and therefore MTBF.
Conventionally, rf power density has been increased while minimizing the overall chip width W by interdigitating small transistor sections or heat-generating regions. These transistor sections are placed as close as possible to each other while maintaining T.sub.active below an acceptable value during device operation.
In high-frequency applications, another problem arises. As the width of the cavity (approximately equal to the chip width, W) in which the transistor is mounted, increases, waveguide resonating modes become possible. These modes effectively feed back a portion of the rf energy to the input of the transistor. This increases the loss of the circuit and can also cause unwanted transistor oscillations. Waveguide resonating modes are strongly attenuated when the width of the cavity is less than half of the effective wavelength, .lambda./2 at the frequency of device operation.
A problem inherent in these conventional interdigitating transistor structures is that adjacent transistor sections contribute significantly to the heat dissipation required by neighboring transistor sections. As a result of demands for increased rf power in system applications at higher and higher frequencies, the standard interdigitated transistor structure is no longer sufficient.
Another conventional approach is to use thinner semiconductor substrates. This increases the dT/dz, where z is in the direction of the substrates thickness, and causes increased heat transfer. However, in many integrated circuit applications, such as monolithic microwave integrated circuits, certain other components such as distributed transmission lines fix lower limits on substrate thickness.
From the above, it can be seen that a need has arisen for a high-frequency rf power transistor having acceptable heat dissipation while being capable of generating increased rf power within fixed physical dimensions.